1. Field of the Invention
The present invention relates to a nonreciprocal circuit element (e.g., an isolator or circulator) for use in a communication appliance such as a cellular telephone or mobile telephone.
2. Description of Background Art
Generally, nonreciprocal circuit elements such as isolators and circulators act to pass signals only in the transmission direction and to block propagation in the opposite direction. These nonreciprocal circuit elements are used in transmitter circuit portions of mobile communication apparatus such as cellular telephones. As these mobile communication apparatus have become smaller, there is an increasing demand for smaller and thinner nonreciprocal circuit elements.
One isolator of this kind has the structure shown in FIGS. 4 and 5. The general structure of the isolator is shown in exploded perspective view in FIG. 4. FIG. 5 is an exploded perspective view of a dielectric multilayer substrate forming a part of the isolator. In the following figures, the surfaces on which elements are mounted face downward. Those portions on which various electrodes are formed by patterning techniques are indicated by shading.
As shown in FIG. 4, this isolator comprises a lower yoke 11 having a bottom wall on which a piece of ferrite 12 is disposed. The dielectric multilayer substrate, indicated by 13, is centrally provided with a recess in which the piece of ferrite 12 is fitted so that the substrate covers the ferrite piece 12. The isolator further includes an upper yoke 15 having a permanent magnet 14 attached to its inner wall surface. The upper yoke 15 is mounted to the lower yoke 11 to form a closed magnetic circuit. The permanent magnet 14 applies a D.C. magnetic field to the ferrite piece 12. The upper yoke 11 and the lower yoke 15 are made of a magnetic metal, and their surfaces are plated with Ag or the like.
This multilayer substrate 13 is fabricated in the following manner. As shown in FIG. 5, a number of dielectric ceramic green sheets having a thickness on the order of tens of micrometers are prepared. Various electrodes are printed on the surfaces of the sheets by patterning or other techniques. These sheets are laminated, pressed against each other, and sintered together, thus forming the multilayer substrate 13. The various electrodes formed in the sheets are connected to each other at desired locations by way of through-holes or via holes.
More specifically, grounding electrodes 1, port electrodes 2a, 2b, and connecting electrodes are formed on sheets 21-26. Port electrode 2c is formed on sheet 51. Thus, input/output portions of the multilayer substrate 13 are formed.
Capacitive electrodes 3a, 3b, and 3c are formed on a sheet 32. The grounding electrodes 1 are formed on sheets 31 and 33, respectively. Matching capacitances connected to respective ends of central electrodes 4a, 4b, and 4c are formed by capacitances created between the capacitive electrodes 3a-3c and the grounding electrodes 1.
Central electrodes 4a, 4b, and 4c are formed on sheets 41, 42, and 43, respectively, such that one central electrode is formed on one respective sheet. The sheets are placed on top of each other in such a way that the central electrodes 4a, 4b, and 4c make an angle of 120 degrees with respect to each other. One end of each of these central electrodes is connected with the corresponding one of the port electrodes 2a, 2b, and 2c. The other ends are connected with the grounding electrodes 1 through via holes.
A terminal resistor R is printed or otherwise formed between the port electrode 2c and the grounding electrode 1 both of which are formed on the rear surface of a sheet 51. The terminal resistor R is overcoated with epoxy resin or other resin.
In the prior art isolator, the central electrodes 4a, 4b, and 4c connected to all the ports have the same strip width and the same strip spacing.
In the structure described above, the respective distances between the central electrodes and the lower yoke (or a grounding surface) or the upper yoke vary from port to port. Therefore, where the central electrodes around the ports are designed to have the same strip width and the same strip spacing as in the prior art techniques, the characteristic impedance of the central electrode differs from port to port. More specifically, the inductance differs from port to port. In consequence, those ports show poor symmetry. Hence, the performance of the isolator deteriorates. Furthermore, the capacitances between the adjacent central electrodes also differ from each other. This further deteriorates the symmetry of the ports.